The preparation of halophenols is customarily carried out by direct halogenation with elemental chlorine and/or bromine. In the case of simple or stable molecules, this appears to be the simplest process. However, when it is applied to more complicated and sensitive structures, it often yields by-products that can seriously interfere with the intended end-use of the halophenols. For instance, U.S. Pat. No. 3,062,781 discloses halogenated diphenols which are obtained by a direct halogenation procedure which require further treatment with sodium hydroxide and triethylamine at 80.degree. C. before they can be converted to polycarbonates of acceptable stability. Such a treatment is necessary in order to remove aliphatically bound halogens formed in the halogenation process. It has been found that these aliphatically bound halogen compounds, which are generally recognizable by their red color, are formed by the cleavage reaction exerted by the hydrogen halide coproduct on the diphenol. For example, when 4,4'-isoproplyidenediphenol (BPA) is employed, the chlorination reaction produces an equimolar amount of hydrogen halide coproduct as shown below wherein X represents a halogen: ##STR1## The HX coproduct produced in (I) above effects a cleavage reaction on BPA or its halogenated derivatives as shown below: ##STR2## Thus, the haloisopropylphenols cause the discoloration and the halogenated phenols also have a disagreeable odor. The formation of by-products is even more pronounced when bromine is employed.
German Patent No. P25 20 317.2 discloses two methods for brominating and/or chlorinating bisphenols; namely, a gas-solid phase method and a suspension method. From these methods, there are obtained a mixture of unreacted bisphenol and statistical mixtures of halogenated bisphenols which are used to prepare polycarbonates having improved fire retardant properties.
In the suspension method disclosed in this German patent, the bisphenol is suspended in a halogen-containing hydrocarbon to produce a halogenated bisphenol. The halogen-containing hydrocarbons disclosed are carbon tetrachloride and tetrachloroethane, carbon tetrachloride being preferred.
While the suspension method disclosed in the above-identified German patent is of interest, it is not entirely satisfactory. For example, since bisphenols are not very soluble in either carbon tetrachloride or tetrachloroethane, excess halogen, i.e., either bromine and/or chlorine, must be used to assure halogenation of the bisphenols. As a result, a significant amount of halogen is lost in the system during the reaction and the rate of halogenation cannot be closely controlled. Thus, this method produces an excess of unreacted bisphenols and, primarily, tri- and tetrahalogenated bisphenols which, when further processed to produce a polycarbonate, do not impart good impact properties to the polycarbonate. Furthermore, the halogenated bisphenols must be isolated from the solvent system before they can be subjected to polymerization to obtain polycarbonates.
Co-pending application Ser. No. 882,191, filed Feb. 28, 1978, and assigned to the same assignee as this case, discloses a process for halogenating diphenols wherein the diphenol is dissolved or suspended in methylene chloride and a halogen gas is metered into the suspension to react with the diphenol. An inert gas is concurrently fed into the system and serves to purge the hydrogen halide produced thereby minimizing the formation of undesirable by-products.
Co-pending application Ser. No. 882,242, filed Feb. 28, 1978, and also assigned to the same assignee as this case, discloses a process for halogenating diphenols wherein the diphenol is dissolved or suspended in methylene chloride and then contacted with sulfuryl chloride. In addition to supplying chlorine, the sulfuryl chloride also reacts with the hydrogen halide produced in a "self-sweeping" reaction thereby minimizing the formation of undesirable by-products.